US7379647B2 - Configuring optical launch powers in optical fiber transmission lines - Google Patents

Abstract

A method includes transmitting optical signals through a heterogeneous sequence of spans of an all-optical transmission line. Each span has an optical transmission fiber connected to an optical amplifier. Each amplifier launches the signals into a sequential remainder of the line. The transmitting includes launching the optical signals into the highest loss fibers with substantially equal average optical launch powers or operating the spans with the highest loss fibers to have substantially equal quality products. The average optical launch powers are substantially equal to the inverse of a sum of (1−T_j)γ_j/[φ_NL·α_j] over the highest loss fibers. The parameters T_j, γ_j, and α_j are the respectiveare, respectively, transmissivity, nonlinear optical coefficient, and loss coefficient of the fiber of the j-th span. The parameter φ_NL is the line's cumulative nonlinear phase shift. Each quality product is the per-span optical signal-to-noise ratio of the associated span times the nonlinear phase shift of the associated span.

Description

BACKGROUND

1. Field of the Invention

The invention relates to optical fiber transmission lines and methods of making and operating such lines.

2. Discussion of the Related Art

Metropolitan and regional communication infrastructures often evolve and grow during their lifetime. Due to their evolution and growth, these infrastructures often eventually combine different technologies. The combinations of technologies can produce optical fiber transmission lines that are span-wise heterogeneous. In such lines, different ones of the optical transmission fibers produce significantly different optical power losses.

In an optical fiber transmission line, an amplifier couples sequentially related optical transmission fibers. The amplifiers produce adequate optical launch powers to compensate for power losses to optical signals in the immediately earlier optical transmission fiber of the line. Conventional amplification schemes provide substantially the same optical launch power to each optical transmission fiber. The constant value of the optical launch power is often found by modeling a generic homogenous optical fiber transmission line to simulate the real optical fiber transmission line.

Unfortunately, this modeling technique does not guarantee a large enough optical signal-to-noise-ratios (OSNRs) at delivery ends of all optical transmission fibers in a real optical transmission line that differs from the generic homogeneous optical fiber transmission line that was used to model the real line. In such real lines, some optical transmission fibers typically deliver optical signals with OSNRs that are too low for reliable further optical transmission. At the delivery ends of such optical transmission fibers, it is usually necessary to add an optical-to-electrical-to-optical (OEO) converter. The OEO converter can include a device that performs error correction on the resulting electrical signal sequence to reduce transmission error levels. After the error correction, the OEO converter optically transmits a partially or completely corrected data stream of optical signals to the next optical transmission fiber of the optical fiber transmission line. Such OEO conversions are typically costly and undesirable in optical fiber transmission lines.

BRIEF SUMMARY

In one embodiment, a method includes transmitting optical signals through a heterogeneous sequence of spans of an all-optical transmission line. Each span has an optical transmission fiber that is connected to an optical amplifier. Each amplifier launches the optical signals into a sequential remainder of the line. The transmitting step includes launching the optical signals into the highest loss fibers with substantially equal average optical launch powers or operating the spans with the highest loss fibers to have substantially equal quality products. The average optical launch powers are substantially equal to the inverse of a sum of (1−T_j)γ_j/[φ_NL·α_j] over the highest loss fibers. The parameters T_j, γ_j, and α_j denote, respectively, the transmissivity, the nonlinear coefficient, and the loss coefficient of the fiber of the j-th span. The parameter φ_NL is the cumulative nonlinear phase shift of the line. Each of the quality products is the per-span optical signal-to-noise ratio of the associated span times the nonlinear phase shift of the associated span.

In another embodiment, a method includes transmitting optical signals through a heterogeneous sequence of optical fiber transmission spans of an all-optical transmission line. Each span has an associated optical transmission fiber connected to an associated optical amplifier. Each amplifier is connected to launch the optical signals into a remainder of the line. The transmitting step includes generating in the highest loss fibers average optical launch powers that produce substantially equal power sums. The power sum of each span is the average optical launch power into the associated fiber plus the average optical power delivered from the associated fiber.

In another embodiment, a method includes transmitting optical signals through a homogeneous sequence of N optical fiber transmission spans of an all-optical transmission line. Each span has an optical transmission fiber connected to an optical amplifier. Each optical amplifier is connected to launch optical signals into a remainder of the line. The transmitting step includes launching the signals in substantially all of the spans with average optical launch powers that are substantially equal to φ_NL·α/[N·γ·(1−T)]. The parameters T, γ, and a respectively, denote the fiber's transmissivity, nonlinear coefficient, and loss coefficient. The parameter φ_NL is the cumulative nonlinear phase shift of the line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of an all-optical transmission line in which the optical amplifiers may, e.g., produce average optical launch powers in a substantially optimum constant output power (OCOP) format or a substantially optimum variable output power (OVOP) format;

FIG. 2 illustrates the optical power evolution in one OVOP embodiment of a heterogeneous all-optical fiber transmission line according to FIG. 1;

FIGS. 3 and 4 plot distributions of maximum reaches for simulated random distributions of spans in optical transmission lines where the optical powers evolve according to fixed output power (FOP), OCOP, and OVOP formats; and

FIGS. 5A-5C illustrate various methods of operating sequences of spans of all-optical fiber transmission lines.

The illustrative embodiments are described more fully by the Figures and detailed description. The inventions may, however, be embodied in various forms and are not limited to embodiments described in the Figures and detailed description.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an all-optical transmission line 10 that includes an optical transmitter 12, an optical receiver 14, and a sequence of optical spans SP₁, . . . , SP_N that form an optical path between an optical amplifier OA₀ at the output of the optical transmitter 12 and an optical amplifier OA_N at the input of the optical receiver 14. Each optical span SP₁, SP₂, . . . , SP_N includes a passive optical transmission fiber OF₁, OF₂, . . . , OF_N whose output end connects to the input of the optical amplifier OA₁, OA₂, . . . , OA_N of the same span SP₁, SP₂, . . . , SP_N. The passive optical transmission fibers OF₁, . . . , OF_N are, e.g., standard single mode fibers (SSMFs), dispersion shifted fibers (DSFs), and/or non-zero dispersion shifted fibers (NZDFs). The output of each optical amplifier OA₀, . . . , OA_N-1 connects to the input end of the optical transmission fiber OF₁, . . . , OF_N of the following span SP₁, . . . , SP_N. The optical amplifiers OA₀, . . . , OA_N provide the optical powers that launch optical signals into the remainder of the all-optical transmission line 10. In particular, the optical amplifier OA₀, . . . , OA_N-1 launch the optical signals into the optical fibers OF₁, . . . , OF_N of the immediately following spans SP₁, . . . , SP_N, and the optical amplifier OA_N launches the optical signals into the receiver 14. The optical amplifiers OA₀, . . . , OA_N may be, e.g., rare-earth amplifiers such as EDFAs or Raman amplifiers and may perform dispersion compensation, e.g., via dispersion compensating fibers. Exemplary optical amplifiers OA₀, . . . , OA_N also include optical add-drop multiplexers and optical cross-connects.

In the all-optical transmission line 10, optical-to-electrical-to-optical (OEO) converters are absent between the optical transmitter 12 and the optical receiver 14. Thus, the sequence of spans SP₁, . . . , SP_N and the optical amplifier OA₀ do not perform OEO conversions. Indeed, the sequence is substantial in length in that it has at least five of the all-optical spans SP₁, . . . , SP_N, e.g., the sequence may have ten or more of such spans SP₁ . . . SP_N. In some embodiments, the optical transmitter 12 and/or the optical receiver 14 may perform such OEO conversions so that the all-optical transmission line 10 is one all-optical segment of a larger non-all-optical fiber transmission line in which one or more OEO converters (not shown) connect separate all-optical transmission segments.

The optical amplifiers OA₀, . . . , OA_N may, e.g., be configured to produce average launch powers for optical signals according to a substantially optimum constant output power (OCOP) format or according to a substantially optimum variable output power (OVOP) format. In the OCOP format, each optical amplifier OA₀, . . . , OA_N produces substantially the same time-averaged and channel-averaged power, P_sys. For example, each of the average launch powers are in the interval P_sys±2 dB, i.e., [0.63P_sys, 1.58 P_sys], or in the smaller interval P_sys±1 dB, i.e., [0.79 P_sys, 1.26P_sys]. In a substantially OCOP format a substantial part of the optical amplifiers OA₀, . . . , OA_N are configured according to an OCOP format, e.g., at least those amplifiers that launch optical signals into the longest 50%, 70%, or 90% of the optical transmission fibers OF₁, . . . , OF_N. In the OVOP format, different ones of the optical amplifiers OA₀, . . . , OA_N produce different average optical powers for launching the same stream of received optical data signals.

Some embodiments of all-optical transmission line 10 have heterogeneous sequences of spans. Herein, a heterogeneous sequence has span optical transmission fibers that are not substantially identical. In particular, the optical transmission fibers of some spans have significantly different lengths and/or are formed of optical fiber with different transmission properties.

Due to the substantially OCOP or OVOP format of the configured average optical launch powers, the optical transmission line 10 can be all-optical over a substantial sequence of spans SP₁, . . . , SP_N. For example, the optical transmission line 10 can be implemented on already installed optical transmission lines to reduce the total number of OEO conversions therein without requiring the re-installation of optical transmission fibers.

All-optical transmission line 10 may implement a substantially OCOP or OVOP format to configure optical amplifiers OA₀, . . . , OA_N to minimize the accumulation of both amplifier stimulated emission (ASE) noise and signal distortions caused by nonlinear optical effects. In heterogeneous optical transmission lines, the substantially OCOP and OVOP formats tend to produce lower bit error rates than in many conventional optical transmission lines. Many of the conventional optical transmission lines are configured to minimize the accumulation of ASE noise without simultaneously limiting the accumulation of the optical signal distortions that are caused by nonlinear optical effects. Other conventional lines are configured to simultaneously minimize both ASE noise and the above-mentioned optical signal distortions based on a fixed optical output power (FOP) format. In the FOP format, average optical launch powers have values that would be approximately optimal for a reference homogeneous all-optical transmission line. Usually, these values are less desirable than the values of the substantially OCOP and OVOP formats, because the reference optical transmission line inadequately models the deployed homogeneous or heterogeneous all-optical transmission line of interest.

To describe the various embodiments of the all-optical transmission line 10, it is useful to introduce several definitions.

First, the definitions include properties that may vary together or individually from span-to-span.

In each optical fiber span SP_j, these properties include an average optical launch power, P_j; an optical transmissivity, T_j; an optical loss, Γ_j; a nonlinear optical coefficient, γ_j; a per-channel nonlinear phase shift, φ^j _NL; and a per-span optical signal-to-noise ratio, OSNR_j. The average optical launch power, P_j is the time-averaged, channel-averaged, per-channel, optical power that is launched into a span's passive optical transmission fiber, OF_j. The transmissivity, T_j, is the fraction of the average optical launch power that is delivered to the output of the span's passive optical fiber OF_j. T_j is approximately equal to exp(−α_j L_j), i.e., ignoring insertion losses, where L_j and α_j are the respective length and loss coefficient per unit length in the optical fiber OF_j. The loss, Γ_j, is equal to −10·Log₁₀(T_j)≅10·α_j·L_j·Log₁₀(e). The nonlinear optical coefficient, γ_j, defines the phase shift, φ^j _NL of an optical signal that nonlinear optical effects cause over the passive optical transmission fiber, OF_j. The per-channel nonlinear phase shift, φ^j _NL, accumulates along the span and is defined by φ^j _NL=₀∫^Ljγ_j P_j(z)dz where P_j(z) is the optical power at point “z” in the optical fiber, OF_j. The per-span optical signal-to-noise ratio, OSNR_j is a time-averaged, per-channel optical signal-to-noise ratio. In particular, OSNR_j in decibels is defined as the ratio of the j-th span's output time-averaged and channel-averaged output signal power over the noise power in a reference bandwidth range B_Ref from the optical amplifier OA_j of the j-th span.

For each optical amplifier OA_j, these properties include an average optical launch power, P_j+1, and a noise figure, F_j. The average optical launch power, P_j+1, is the power that the time-averaged per-channel, optical power that the optical amplifier OA_j launches into the remainder of the transmission line 10, e.g., the following span's optical fiber, OF_j+1. Below, the vector of average optical launch powers, {P₁, . . . , P_N}, will often be written as an N-component vector, P. The noise figure, F_j, is a characteristic of the optical amplifier OA_j alone. The noise figure, F_j, is given as the OSNR at the output of the optical amplifier, OA_j, divided by the OSNR at the output of the optical amplifier, OA_j, for a shot-noise limited input optical signal.^{1 1} An alternate definition of the noise figure, F_j, of optical amplifier, OA_j, is OSNR at the input of the optical amplifier, OA_j, divided by the OSNR at the output of the optical amplifier, OA_j.

Second, the definitions include constants and properties of the entire all-optical transmission line 10.

These constants and properties include a reference bandwidth, B_Ref, Plank's constant, h, a selected channel frequency, v, a delivered optical signal-to-noise ratio, OSNR_Del, a cumulative nonlinear phase shift, φ_NL and a system nonlinear phase shift, φ_sys. The constant reference bandwidth, B_Ref, is 0.1 nanometers (nm). The channel frequency, ν, is the optical transmission frequency for the channels in the all-optical transmission line 10. The delivered OSNR, OSNR_Del, is a ratio of the signal power over the noise power that the all-optical transmission line 10 delivers in the reference bandwidth, B_Ref, to the optical receiver 14 in response to receiving a non-shot limited input optical signal power. For such input optical signal powers, the delivered OSNR is approximately given by:

$1/{\mathrm{OSNR}}_{\mathrm{Del}}=\sum _{j=1}^{N}1/{\mathrm{OSNR}}_j=\sum _{j=1}^N\left({\mathrm{hvB}}_{\mathrm{Ref}}{F}_j\right)/\left(T_j{P}_j\right).$
The value of OSNR_Del provides a measure of cumulative ASE noise over the entire all-optical transmission line 10. The cumulative nonlinear phase shift, φ_NL, is the sum of optical phase shifts, φ^j _NL, produced by nonlinear effects in the sequence of spans SP₁, . . . , SP_N of the all-optical transmission line 10. If splice losses are ignored, the cumulative nonlinear optical phase shift, φ_NL, is approximately given by:

${\varphi }_{\mathrm{NL}}=\sum _{j=1}^N{\varphi }_{\mathrm{NL}}^j=\sum _{j=1}^N\left({\gamma }_j/{\alpha }_j\right)P_j\left(1-T_j\right).$
The value of φ_NL provides a measure of optical signal distortion caused by nonlinear optical effects over the entire all-optical transmission line 10. The system nonlinear phase shift, φ_sys, is a selected upper limit on the cumulative nonlinear optical phase shift, φ_NL, e.g., φ_sys≧φ_NL. Above, sums are over all the N spans SP₁, . . . , SP_N in the all-optical transmission line 10.

Both OCOP and OVOP embodiments of all-optical transmission line 10 configure the optical amplifiers OA₀, . . . , OA_N to produce average optical launch powers that approximately maximize OSNR_Del while simultaneously constraining φ_NL.

In OCOP embodiments, each optical amplifier OA₀, . . . is configured to produce substantially the same time-averaged and channel-averaged, per-channel, optical power, P_sys, for launching optical pulses into the spans' optical fibers OF₁, . . . , OF_N. From the above formulas, the delivered OSNR and cumulative nonlinear optical phase shift are approximately: An alternate definition of the noise figure, F_j, of optical amplifier, OA_j, is OSNR at the input of the optical amplifier, OA_j, divided by the OSNR at the output of the optical amplifier, OA_j.

${\mathrm{OSNR}}_{\mathrm{Del}}={P_{\mathrm{sys}}\left[\sum _{j=1}^N{\mathrm{hvB}}_{\mathrm{Ref}}{F}_j/T_j\right]}^{-1},\mathrm{and}$ ${\varphi }_{\mathrm{NL}}=P_{\mathrm{sys}}\sum _{j=1}^N{\gamma }_j\left[1-T_j\right]/{\alpha }_j,\mathrm{respectively}.$
Solutions of the above equations provide the values of P_sys and OSNR_Del in the OCOP embodiments. Such solutions typically exist for an OSNR_Del greater than a selected minimum value if the number of spans, N, is small enough.

The OCOP equations simplify for homogeneous embodiments of the optical transmission line 10. In these embodiments, all spans have the same type of optical transmission fiber and all optical amplifiers are of the same type. Thus, the span and amplifier properties α_j, γ_j, T_j, and F_j take the respective span-independent values α, γ, T, and F. In such homogeneous OCOP embodiments, each span SP_j will be configured to have an average optical launch power substantially equal to P_sys, wherein P_sys=φ_NL·α/[N·γ·(1−T)]≅φ_NL·α/[N·γ]. The last approximate relation follows, because the fibers' transmissivity, T, is typically very small compared to one, e.g., less than about 0.05.

Substantially OCOP embodiments provide other embodiments of all-optical transmission line 10. In such embodiments, some average optical launch powers may differ from the value of P_sys for two reasons. First, in heterogeneous embodiments, the exact equation for φ_NL receives only small contributions from spans in which average optical launch powers are very low, e.g., ones of the spans SP₁, . . . , SP_N with low losses in the associated passive optical transmission fibers OF₁, . . . , OF_N. Thus, the special spans SP₁, . . . , SP_N that average launch optical power into the highest loss ones of the fibers OF₁, . . . , OF_N may be the only spans that need provide average optical launch powers substantially equal to P_sys. The optical amplifiers OA₀, . . . , OA_N that launch optical signals into lower loss ones of the fibers OF₁, . . . , OF_N may, e.g., be configured to produce significantly lower average optical launch powers. The highest loss ones of the optical transmission fibers may, e.g., have losses within 6 dB or within 9 dB of the loss of the highest loss one of the optical transmission fiber OF₁, . . . , OF_N. An all-optical transmission line 10 may have five or more or even ten or more of the special spans with the highest loss ones of the optical transmission fibers. Second, in homogeneous and heterogeneous OCOP embodiments, the substantially equal average optical launch powers may also be distributed over the interval of P_sys ±2 dB, i.e., [0.63 P_sys, 1.58 P_sys]or over the smaller interval of P_sys ±1 dB, i.e., [0.79P_sys, 1.26P_sys]. Any combination of the above-described variations provides a substantially OCOP embodiment with potentially advantageous properties.

In OVOP embodiments, the optical amplifiers OA₀, . . . , OA_N are configured to produce average optical launch powers P₁, . . . , P_N that may vary from span-to-span. In the OVOP embodiments, determining the average optical launch power vector, P, involves minimizing OSNR_Del subject to a constraint that the cumulative nonlinear optical phase shift, φ_NL is equal to the preselected system value φ_sys, e.g., φ_sys=1 radian. Such a constrained minimization may be performed by finding the minimum of a function L(P, λ) that depends on a Lagrange multiplier λ. In particular, L(P, λ) is given by:

$L\left(P,\lambda \right)={\mathrm{hvB}}_{\mathrm{Ref}}\sum _{j=1}^N{F}_j/\left(T_j{P}_j\right)+\lambda \left[\sum _{j=1}^N\frac{{\gamma }_j}{{\alpha }_j}{P}_j\left(1-T_j\right)-{\varphi }_{\mathrm{sys}}\right].$
The Lagrange multiplier, λ, imposes the constraint φ_NL=φ_sys when the function L(P, λ) is minimized with respect to λ. Nevertheless, it is more convenient to minimize L(P, λ) by first finding its extrema with respect to the average optical launch powers, i.e., by requiring that ∂L(P,λ)/∂P_j=0 for all spans “j”. Afterwards, the extrema equations provide a value for the Lagrange multiplier λ. In particular, the extrema equations become:

$\lambda =\frac{{\mathrm{hvB}}_{\mathrm{Ref}}{F}_j{\alpha }_j}{{{\gamma }_j\left(P_j\right)}^2{T}_j\left(1-T_j\right)}=\frac{1}{{\mathrm{OSNR}}_j·{\varphi }_{\mathrm{NL}}^j}=\frac{1}{{\mathrm{OSNR}}_{\mathrm{Del}}·{\varphi }_{\mathrm{NL}}}.$
Each span's quality product is defined by the product OSNR_j·φ_NL ^j. In an OVOP embodiment, the spans' quality products are substantially equal to OSNR_Del·φ_NL for substantially all spans of the all-optical transmission line 10. In particular, the spans' quality products may be distributed over the interval [0.63·OSNR_Del·φ_NL, 1.58·OSNR_Del·φ_NL] or over the smaller interval [0.79·OSNR_Del·φ_NL, 1.26·OSNR_Del·φ_NL].

Minimizing the function L(P, λ) also provides expressions for the average optical launch powers {P₁, . . . , P_N} and the multiplier, λ, in terms of span properties and the system's cumulative nonlinear phase shift, φ_sys, as follows:

$P_j={\varphi }_{\mathrm{sys}}{\sqrt{\frac{F_j{\alpha }_j}{{\gamma }_j{T}_j\left(1-T_j\right)}}\left[\sum _{i=1}^N\sqrt{\frac{F_i{\gamma }_i\left(1-T_i\right)}{{\alpha }_i{T}_i}}\right]}^{-1},\mathrm{and}$ $\lambda ={\frac{{\mathrm{hvB}}_{\mathrm{Ref}}}{{\left({\varphi }_{\mathrm{sys}}\right)}^2}\left[\sum _{j=1}^N\sqrt{\frac{F_j{\gamma }_j\left(1-T_j\right)}{{\alpha }_j{T}_j}}\right]}^2.$
The above expression describes how to configure optical amplifiers OA₀, . . . , OA_N to produce the OVOP embodiments of the all-optical transmission line 10.

In some OVOP embodiments, all spans SP₁, . . . , SP_N use the same type of optical transmission fiber and the optical amplifiers OA₀, OA₁, . . . , OA_N of the line 10 are also of the same type. Then, the OVOP expressions for the P_j's simplify. In particular, (α_j, γ_j)=(α, γ) for each span SP_j, and F_j=F for each amplifier OA_j of such an embodiment. Assuming also that the T_j's are small compared to one, the OVOP expressions for the P_j's of these embodiments become the constraints:

$P_j-\frac{{\mathrm{<figure-callout\; id="2"\; label="\Gamma \; j"\; filenames="US07379647-20080527-D00002.png"\; state="\{\{state\}\}">\Gamma </figure-callout>}}_j}{2}=P_{\mathrm{average}}-\frac{{\mathrm{<figure-callout\; id="2"\; label="\Gamma \; average"\; filenames="US07379647-20080527-D00002.png"\; state="\{\{state\}\}">\Gamma </figure-callout>}}_{\mathrm{average}}}{2}.$
Here, P_average and F_average are averages of P_j and Γ_j, respectively, over the spans SP₁, . . . , SP_N of the all-optical transmission line 10, i.e., P_average=Σ_j P_j/N and Γ_average=Σ_jΓ_j/N.

FIG. 2 illustrates an interpretation of the above-stated constraints on P_j−Γ_j/2. The constraints are equivalent to the average launch optical power plus the time-averaged and channel-averaged delivered optical power being substantially equal to (2 P_average−Γ_average) for each span SP₁, . . . , SP_N in the all-optical transmission line 10. For the various spans SP₁, . . . , SP_N, the sums may be distributed over the interval of (2 P_average−Γ_average)±2 dB, i.e., [0.63·(2 P_average−Γ_average), 1.5 8·(2 P_average−Γ_average)] or may be distributed over the smaller interval of (2 P_average−Γ_average)+1 dB, i.e., [0.79·(2 P_average−Γ_average), 1.26·(2 P_average−Γ_average)].

Substantially OVOP embodiments of heterogeneous all-optical transmission line 10 only impose the above-recited conditions on a proper subset of the spans SP₁, . . . , SP_N. In particular, the conditions may be imposed on only the special spans in which the optical transmission fibers OF₁, . . . , OF_N have the highest losses. For example, the special spans may include spans with an optical transmission fiber whose loss is within 6 dB or within 9 dB of the loss of the highest loss transmission fiber fibers OF₁, . . . , OF_N. Then, P_average and Γ_average may be averages over the subset composed of these highest loss optical fibers. A substantially OVOP embodiment of the all-optical transmission line may have at least five of such special spans with highest loss fibers or may have ten or more of such special spans.

To evaluate relative performances, a simulation was performed to compare a distribution of optical transmission lines configured according to the FOP, OCOP, and OVOP methods. Each optical transmission line was constructed by selecting a sequence of total span losses, i.e., Γ₁, Γ₂, Γ₃, . . . , in a pseudo-random manner. In the simulated lines, optical amplifiers had 6 dB noise figures, and optical transmission fibers had span loss coefficients of 0.2 dB per kilometer and nonlinear optical coefficients of 1.27 per watt-kilometer. In the simulated lines, the FOP method configured the average optical launch powers to 3.0635 dBm. These values were found by optimizing a reference homogeneous optical transmission line with 18 spans of 100 km long optical transmission fibers and per-span losses of 20 dB. In the simulations, the OCOP and OVOP methods configured average optical launch powers as already described and neglected inter-span connector losses.

FIGS. 3 and 4 plot maximum reaches of two sets of 1,000 simulated optical transmission lines. The loss distributions were between 13 dB and 33 dB for the spans' passive optical transmission fibers in FIG. 3 and were between 8 dB and 28 dB for the spans' passive optical transmission fibers in FIG. 4. Maximum reaches were defined as the lengths of the sequences of spans for which OSNR_Del≧19 dB and φ_NL≦1 radian.

In both FIGS. 3 and 4, the simulated optical transmission lines appear to have substantially larger maximum reaches when configured according to the OCOP method rather than according to the FOP method. Indeed for both distributions of optical transmission lines, peak values of maximum reaches appear to be 20-40% larger when the average optical launch powers are configured according to the OCOP method.

In both FIGS. 3 and 4, the simulated optical transmission lines appear to have even larger maximum reaches when configured according to the OVOP method rather than according to the OCOP method. Both loss distributions for the spans' passive optical transmission fibers produce peak values of maximum reaches that appear to be at least 40% larger when the average optical launch powers are configured according to the OVOP method rather than according to the FOP method.

FIGS. 3 and 4 show that many heterogeneous optical transmission lines will need less OEO conversions when configured according to the OVOP or OCOP methods than when configured according to the FOP method. Thus, using the OVOP or OCOP methods to adjust optical power evolutions in deployed metro and regional optical transmission lines should significantly reduce the need for costly OEO converters therein.

FIG. 5A illustrates a method 20 of operating an all-optical fiber transmission line, e.g., an embodiment of the line 10 of FIG. 1, to transmit a stream of data. The method 20 includes transmitting optical signals through a heterogeneous sequence of spans of the all-optical fiber transmission line (step 22). The transmitting step includes either launching the optical signals into highest loss passive optical transmission fibers of the sequence with average optical launch powers that are substantially equal to a special sum or operating the spans with the highest loss passive optical transmission fibers to have substantially equal quality products. Here, each average optical launch power is the time-average and channel-average of the optical launch power in the associated span. The special sum adds (1−T_j)γ_j/[φ_NL·α_j] over the highest loss ones of the passive optical transmission fibers. Each quality product is the per-span optical signal-to-noise ratio of the associated span times the nonlinear phase shift of the associated span. For example, five or more of the spans of the sequence may have one of the highest loss passive optical transmission fibers. The method 20 may, e.g., implement the above-described substantially OCOP format or the above-described substantially OVOP format.

FIG. 5B illustrates another method 30 of operating an all-optical fiber transmission line, e.g., an embodiment of the line 10 of FIG. 1, to transmit a stream of data. The method 30 includes transmitting optical signals through a heterogeneous sequence of spans of the line (step 32). The transmitting step includes generating in highest loss passive optical transmission fibers of the sequence average optical launch powers that produce substantially equal power sums. Here, each average optical launch power is the time-average and channel-average of the launch power in the associated span. For each span, the power sum is the average launch optical power in the associated passive optical transmission fiber plus the average delivered optical power from the associated passive optical transmission fiber. Five or more of the spans may have one of the highest loss passive optical transmission fibers. The method 30 may, e.g., use a substantially OVOP format as already described.

FIG. 5C illustrates another method 40 of operating a homogeneous all-optical fiber transmission line, e.g., an embodiment of the line 10 of FIG. 1, to transmit a stream of data. The method 40 includes transmitting optical signals through a homogeneous sequence of N spans of the line (step 42). The transmitting step includes launching the optical signals in substantially all of the passive optical transmission fibers of the spans of the sequence with average optical launch powers that are substantially equal to φ_NL·α/[N·γ·(1−T)]. For example, substantially all of the spans may include 70% or more, 80% or more, 90% or more, or all of the spans. Here, the average optical launch power is the time-average and channel-average of the launch power in the passive optical transmission fiber of the span. The parameters T, γ, and α are the respective transmissivity, nonlinear coefficient, and loss coefficient of the fibers. The method 30 may, e.g., use a substantially OCOP format as already described.

From the above disclosure, the figures, and the claims, other embodiments will be apparent to those of skill in the art.

Claims (7)

1. A method, comprising:

transmitting optical signals through a heterogeneous sequence of optical fiber transmission spans of an all-optical transmission line, each span having an optical transmission fiber connected to an optical amplifier, each amplifier being connected to launch the optical signals into a sequential remainder of the line; and

wherein the transmitting comprises either launching the signals into highest loss ones of the fibers with substantially equal average optical launch powers or operating the spans with the highest loss ones of the fibers to have substantially equal quality products; and

wherein the average optical launch powers are substantially equal to the inverse of a sum of (1−T_j)γ_j/[φ_NL·α_j] over the highest loss ones of the fibers, the parameters T_j, γ_j, and α_j being the respective transmissivity, nonlinear coefficient, and loss coefficient of the fiber of the span indexed by j, the parameter φ_NL being the cumulative nonlinear phase shift of the line, and each quality product is the per-span optical signal-to-noise ratio of the associated span times the nonlinear phase shift of the associated span.

2. The method of claim 1, wherein the highest loss ones of the fibers include the fibers of at least five of the spans.

3. The method of claim 2, wherein the transmitting comprises launching the optical signals into the highest loss ones of the fibers with time and channel averaged powers of between 0.63 times the fixed value and 1.58 times the fixed value.

4. The method of claim 1, wherein the highest loss ones of the fibers include the fibers of the transmission spans whose losses are within 6 decibels of the highest loss of one of the fibers of the transmission spans.

5. The method of claim 1, wherein the operating spans with the highest loss ones of the fibers have quality products of between 0.63 times a selected value and 1.58 times the selected value.

6. The method of claim 5, wherein the selected value is substantially equal to a delivered OSNR of the sequence in decibels times a cumulative nonlinear phase shift of the sequence.

7. The method of claim 3, the transmitting launches the optical signals into the highest loss ones of the fibers with time and channel averaged optical powers of between 0.79 times the fixed value and 1.26 times the fixed value.

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